Typically, the surface of an electrical steel sheet has an insulating coating for reducing eddy current loss, and the insulating coating is required to have coating characteristics such as corrosion resistance and adhesion, in addition to the insulating properties.
Those that are known as such insulating coatings include an insulating coating containing a phosphate, an organic resin, and the like as main components, an insulating coating containing silica, an organic resin, and the like as main components, an insulating coating containing silica, a phosphate, and the like as main components, and an insulating coating containing alumina, boric acid, and the like as main components. An insulating coating is typically formed by applying a solution containing these components to a surface-treated silicon steel sheet, and baking it.
However, when this solution is allowed to stand, the viscosity may increase due to agglomeration or the like, which may cause problems during coating, or have an adverse effect on the characteristics of the insulating coating.
Patent Document 1 discloses using a mixture of an alumina sol, boric acid, and nitric acid as an insulating coating for an electrical steel sheet. This document describes that the viscosity stability of the coating solution can be ensured by using an alumina sol having specific properties, and setting the molar ratio between the alumina sol and boric acid within a specific range.
Patent Documents 2 to 7 disclose insulating coatings for electrical steel sheets each containing silica, a phosphate, and the like as main components. These documents describe that excellent performance can be imparted to the insulating coatings to be formed, by adding specific metal compounds, limiting the proportions of the components, or using specific phosphates, for example.
However, in an insulating coating for an electrical steel sheet containing silica, a phosphate, and the like as main components, no proposal has been made to improve the viscosity stability of the coating solution for forming the insulting coating.